1. Field of the Invention
The present invention generally relates to separation of fluids of different densities and, more particularly, to separators for immiscible fluids such as oil and water.
2. Description of the Prior Art
Since the earliest applications of mechanical motive power systems to water-borne vessels, particularly serious problems have existed in regard to fluids collecting in the lower portions of the hulls of such vessels. The very fact that hulls of water-borne vessels must be fluid tight makes the hulls capable of collecting and holding fluids in lower portions thereof, generally referred to as the bilge. A major portion of fluids collecting in the bilge consists of water which may derive from condensation, leakage, spray, waves breaking over portions of the hull and the like. These fluids have the effect of reducing buoyancy of the hull and must be removed periodically or continuously in order to preserve the performance and safety of the vessel. If only water collects in the bilge, it can merely be discharged overboard by bailing or pumping.
However, particularly when mechanical motive power arrangements are provided for the vessel, additional problem are encountered. Specifically, such motive power systems invariably involve the use of at least petroleum-based lubricants and usually petroleum based fuels such as gasoline and/or fuel oils, hereinafter referred to as "oil". Some release of these materials within the vessels is virtually unavoidable, causing these materials to be similarly collected within the bilge. Since petroleum-based materials are less dense than water and generally immiscible therewith, they will usually rise to the top of water collecting in the bilge and thus exposed to the ambient atmosphere. The flammability of such materials, particularly in proximity to sources of ignition presented by the motive power system (e.g. steam plants, and internal combustion engines such as ignition and diesel engines) presents a major fire hazard in addition to reducing the buoyancy of the vessel.
The presence of oil with water within the bilge also presents difficulties of disposal of the bilge fluids. Specifically, the discharge of petroleum-based material into the environment has a well-recognized deleterious effect on bodies of water since oil will tend to form a thin layer over the surface thereof which impedes gas transfer at the water surface, as well as other effects. Therefore, it is imperative that little or no oil be discharged with water from the bilge. Of several possible approaches to reduction of oil discharge, most have proven impractical. For example, merely pumping all bilge fluids to a holding tank may effectively reduce the fire hazard presented by the oils while eliminating discharge from the vessel. However, such storage does not restore buoyancy of the vessel since the fluids are retained on board and usually at a higher location within the vessel where center of gravity and vessel trim and stability will be adversely affected. Similarly, while reasonably good separation of the oil from the water may occur in the relatively undisturbed confines of the bilge, merely drawing off the water does nothing to alleviate the fire hazard presented by the oils. The varying levels of the oils inhibit the effective pick-up of oil for storage without also requiring the storage of excessive and impractical amounts of water, as well, in the holding tanks.
Accordingly, it has been the practice to provide oil/water separators aboard vessels to provide a more complete separation in the bilge. By the use of such oil-water separators, the amount of water included with the oil pumped to holding tanks can be reduced to satisfactory levels consistent with vessel performance and the amount of oil included with water to be jettisoned may be held to acceptable levels.
Unfortunately, wide variations in efficiency of separation of oil and water has been encountered in different designs of oil/water separators, hereinafter referred to by the acronym OWS, particularly as a matter of scale. When bilge fluids are pumped, a mixing action occurs and the oil, though immiscible with the water, is dispersed into fine particles or droplets. These fine droplets do not separate as easily from the water due to the relatively large surface area and reduced volume. Any further forced motion of the dispersion will also tend to maintain the dispersion and inhibit separation of the oil and water. Nevertheless, some degree of slow fluid motion is necessary to cause contact between oil droplets which then merge and more readily separate from the water. This effect can be enhanced by the provision of stacks of perforated undulating plates which tend to direct the motion of oil droplets and statistically increase the likelihood of contact between the droplets. Such plates are commercially available from several suppliers, such as General Electric Co.
While the undulating shape presents a resistance to flow of fluids between the plates, the combination of the area of plates possible within a given volume at preferred spacings, together with the degree to which the flow rate of pumped bilge fluids can be slowed during passage through a given volume of an OWS and the affinity of oil for the surface of the polypropylene material of the plates, while yielding good results in very large separators, does not provide acceptable results in separators of smaller volume required for smaller vessels. More specifically a total maximum flow rate of about four gallons per minute is considered to define a low flow separator. However, the actual velocity of fluids at various points within the separator determines the efficiency with which separation will occur. Accordingly, it can be readily understood that the fluid velocity cannot easily be kept low in separators of small total volume. As a result, smaller low flow rate separators are typically characterized by lower oil and water separation efficiencies. There also appear to be scaling effects since even if separator volume and flow rate are diminished by the same factor, reduction of efficiency is generally observed.
It should be further understood that separation of oil and water can be effectively done by mechanisms other than gravity. For example, centrifugal separators are well-known for increasing the forces tending to separate fluids of different densities. However, failure of seals, noise and safety concerns in regard to high speed rotating machinery makes centrifugal separators unsuitable for shipboard use. Further, the amount of power consumed by such mechanisms reduces the amount of power available for other shipboard uses (or, if the separator is self-powered, increases vessel weight) which often must be closely budgeted on small vessels. Therefore, for shipboard use on small vessels, passive oil and water separators are much to be preferred if separation efficiencies comparable to large passive separators can be achieved at separator weights and volumes which can be accommodated.